In the aforesaid copending application, there is shown and described a recirculating air impingement heater-dryer which is utilized for heating of recycled asphalt pavement (RAP). In this device air at a high velocity is inserted into the layer of RAP on top of a conveyor belt which passes beneath air impingement tubes. The high velocity air removes water from the surface of the RAP by causing rapid evaporation of the moisture. The high velocity air also forms wind row lines of RAP on the conveyor belt. The creation of the wind rows is provided by the high velocity air, and exposes different surfaces of the RAP as the material is moved back and forth from row to row by the use of the tubes. It has been found that this is an extremely efficient method for removing water vapor from RAP and for heating it.
The dryer also includes a recirculating feature which permits air to be recirculated within the dryer, and new air added as oxygen is required by the burners. Old air and moisture are removed from the recirculating gases as new oxygen and heat is required by the system.
In the art of recycling asphaltic pavement, it has been found that a process whereby new virgin rock is mixed with old asphaltic pavement is desirable. In this process, the rock may be heated to a temperature in excess of 600.degree. F. and then the rock may be mixed with RAP. The high heat of the rock combines with the low heat of the RAP to produce a uniform asphaltic mixture which as a temperature which is a function of the average heat count of the RAP and the rock. In this method utilizing RAP at plant operating site temperatures and virgin rock at 600.degree. F. or more, it has been found that the RAP of the final mixture cannot exceed approximately 40%. This limitation is undesirable because it is often desirable to have a much higher percentage RAP content, so that there will not be any excess RAP when the recycling and paving operations are complete.
In U.S. Pat. No. 4,481,039 Mendenhall there is shown an apparatus for heating both RAP and virgin rock in a single rotating drum which includes a burner 40. The drum receives RAP from conveyor 26 through chute 16 and heats it in a cold section 28 of the drum. Virgin rock may also be added at hopper 30 and heated in the hot end of the drum 12. The hot rock from hot end 12 is deposited on conveyor 55 and may also be added to the virgin RAP being added to the cold end of the drum from conveyor 26. The final product which is a mixture of RAP and preheated virgin rock is then removed from the process by a conveyor 60. In this system the RAP is heated primarily by the addition of hot rock at hopper 26, and also by the flame from burner 40 which exhausts at pipe 22. By this process, the temperature of the virgin RAP is raised by the heat from the gases of burner 40 as well as the heat contained in the stone which is removed from the drum at conveyor 44.
Mendenhall also provides for a separate return path of hot rock 55 to conveyor 35 which provides for temperature control of the rock heating section of the drum because of the feedback of heated material.
U.S. Pat. No. 3,999,743 Mendenhall shows another rotating drum apparatus which receives three different grades of asphaltic particles or rock for the purpose of providing different heating rates to different aggregate sizes.
U.S. Pat. No. 4,095,284 Mendenhall also shows a mixing device which heats different aggregate sizes for different times and rates in order to achieve uniform heating throughout.